A number of systems for converting sunlight to electricity are known. One such system that has proven useful in efficiently producing electricity from the sun's radiation is described in U.S. Pat. No. 4,691,076. In that system, an array is formed of semi-conductor spheres. Each sphere has a P-type interior and an N-type skin. A plurality of the spheres are housed in a pair of aluminum foil members which form the contacts to the P-type and N-type regions. The foils are electrically insulated from one another and are flexible. Multiple arrays can be interconnected to form a module of solar cell elements for converting sunlight into electricity.
In order to produce sufficient quantities of the arrays, it is necessary to have a process for their manufacture that is uncomplicated, low cost and efficient. An uncomplicated system would be one using currently available technology constructed in such a way that the applicable process steps can be conducted in a highly repeatable manner. Moreover, the less complicated the process steps, generally the more cost effective will the entire process be carried out. Finally, the more repeatable the process, the more efficiently the operation and the higher production of solar arrays.
A key process step in the making of silicon solar cells is the ability to introduce controlled quantities of dopant impurity atoms into the silicon. In one widely used method of introducing impurities into the crystal, the impurity is delivered to the surface in vapor form diluted to the proper concentration with an inert carrier gas such as nitrogen. While this method works quite well for planar solar cells, it has significant disadvantages when applied to spheral solar cells.
One significant problem with using vapor deposition for diffusing silicon spheres is the difficulty in obtaining a uniform diffusion depth. The spheral shape and small size (0.0175 inches in diameter) make it very difficult to prevent the spheres from touching either other spheres or the quartz diffusion boats. If the spheres do touch one another or the quartz boat, that part of the sphere would be shielded from the doping gas resulting in a hondiffused area and an electrically shorted sphere. The entire sphere surface must be doped, not just the frontside as in planar solar cells and integrated circuit components. This shielding effect would also limit the amount of spheres diffused in a single run.
Past attempts at obtaining nonshorted spheres have included the rotation of the spheres during the vapor deposition process. This rotation did help, but resulted in nonuniform diffusion profiles and did not completely eliminate electrically shorted spheres. Nonuniform diffusion profiles will also cause a variation in electrical properties from sphere to sphere.
In an N on P solar cell, POCl.sub.3 is the preferred choice as the N-type dopant due to its phosphorus concentration vs. depth profile near the surface. Unfortunately POCl.sub.3 becomes tacky during a diffusion run and will deposit onto the walls of the quartz furnace tube. This residue POCl.sub.3 must be removed regularly to maintain cleanliness and process control.
Tests have also shown that static electricity causes the spheres to adhere to the sides of the rotating cylinder preventing the spheres from rotating and receiving a uniform diffusion.
It is also difficult with the vapor deposition process to reliably obtain the shallow junction depths (0.2-0.3 microns) necessary for generating high currents and correspondingly high solar-to-electrical efficiencies. The total diffusion time, approximately 5-8 minutes at 850.degree. C.) involved in doing shallow junctions is too short for optimal control and repeatability. The diffusion time becomes even shorter at higher temperatures which further complicates the control problem.